This invention relates to a method and apparatus for adjusting the shape of components of the eye and more particularly to making fixed changes in the corneal curvature to correct refractive error.
Deviations from the normal shape of the corneal surface produce errors of refraction in the visual process. The eye in a state of rest, without accommodation, focuses the image of distant objects exactly on the retina. Such an eye enjoys distinct vision for distant objects without effort. Any variation from this standard constitutes ametropia, a condition in which the eye at rest is unable to focus the image of a distant object on the retina. Hyperopia is an error of refraction in which, with the eye at rest, parallel rays from distant objects are brought to focus behind the retina. Divergent rays from near objects are focused still further back. In one aspect of hyperopia, the corneal surface is flattened which decreases the angle of refraction of rays as they pass through the refractive surfaces of the cornea, causing a convergence or focus of the rays at a point behind the retina. The retina is comprised partially of nerve fibers which are an expansion of the optic nerve. Waves of light falling on the retina are converted into nerve impulses and carried by the optic nerve to the brain to produce the sensation of light. To focus parallel rays on the retina, the hyperopic eye must either accommodate, i.e., increase the convexity of its lens, or a convex lens of sufficient strength to focus rays on the retina must be placed before the eye.
Myopia is that refractive condition in which, with accommodation completely relaxed, parallel rays are brought to focus in front of the retina. One condition which commonly causes myopia is when the corneal curvature is steepened, thus the refraction of rays is greater as the rays pass through the refractive surfaces of the cornea, and the over-refracted rays converge or focus in front of the retina in the vitreous of the eye. When the rays reach the retina they become divergent, forming a circle of diffusion and consequently a blurred image. A concave lens is used to correct the focus of the eye for myopia.
The normal treatment of these classic forms of refractive error of the eye is with the use of eyeglasses or contact lenses, both of which have well-known disadvantages to the user. It has been estimated that 60 million pairs of eyeglasses and 3 million pairs of contact lens are sold annually.
Recent research has been directed to operative techniques to change the refractive condition of the eye. Such techniques are generally referred to as "keratorefractive techniques". Two such techniques are more particularly called keratophakia and keratomileusis. Keralomileusis involves the regrinding of a corneal lamella into a meniscus or hyperopic lens to correct myopia or hyperopia. A corneal optical lathe has been especially developed for this procedure and is also used in the keratophakia procedure, when a homograft ground into a convex lens is placed interlamellarly to correct aphakic hypermetropia. The homograft tissue (corneal lamella) is frozen with carbon dioxide. The homograft is cut as a contact lens would be, i.e., to the optical power required to effect the desired optical correction of the cornea. In keratomileusis, the anterior corneal lamella is shaped by the lathe and in keratophobia, it is the corneal stroma of a donor eye that is shaped by the lathe. These techniques have a broad application in the correction of high hyperopic and myopic errors. These procedures require radial cutting of the cornea about the periphery of the graft which weakens the cornea so that pressure from fluids below the incisions pushes up under the cuts and flattens the curvature of the cornea. This flattening of the cornea results in refractive errors to the eye not compensated for by the graft. Suturing in these operations also causes radial asymmetry of the cornea which consequently promotes astigmatic error in this regard. Sutures also cause scarring of the corneal tissue, which scar tissue loses its transparency. Surgical correction of astigmatism is accomplished by asymmetrically altering the corneal curvatures. The effect of a peripheral distorting force may be easily visualized by imagining an inflated balloon with a spherical surface being compressed between the palms of the hands. Because the volume of air in the balloon is constant, the surface area remains constant. The previously spherical anterior surface is distorted meridional as a result of compressing the diameter between the hands so that the curvature changes without changing the circumference of the surface. The meridian passing over the balloon between the extended fingers steepens, while the uncompressed meridian at right angles thereto flattens as its diameter lengthens in proportion to the shortening of the compressed diameter. This demonstrates the effect that may result from slight variations in the symmetrical patterns or intentional asymmetrical patterns attempted to be accomplished during surgical procedures and attendance suturing. It is thus seen that present procedures in keratorefractive techniques are best limited to situations where other more standard corrective practices are found ineffective. It is readily seen that the limiting factors in such surgical techniques is the gross complexity involved not only with multiple incisions in corneal tissue for affecting the procedures but also complex suturing patterns, resulting in gross restructuring of the eye. The eye is thus faced with a difficult job of adjusting to this trauma.
Over the past few years developments have been made in the use of lasers as a means to reshape the cornea in an attempt to get rid of refractive errors. In these processes, pulsed lasers remove tissue from the cornea by shaving off or vaporizing portions of the corneal surface to cause it to flatten. The most common type is an Exemer laser. The fundamental effect of such a laser on tissue is a photochemical one, the breaking of molecular bonds with so much energy that the tissue fragments fly from the surface at supersonic speeds, leaving behind a discreet space. The process has been designated as ablative photodecomposition or photoablation.
The use of Exemer lasers require delivery of the beam to the eye in a controlled manner requiring that the homogenous beam be appropriately managed and focused because the optical elements must withstand the high energy photons and because the beam must be shaped to a non-uniform configuration to create the new non-uniform optical surface of the cornea. Such delivery system contains multiple components including lenses to expand or focus the beam, mirrors to direct the beam, modulators to homogenize the beam, masks to shape the beam, and detectors to measure the intensity and configuration of the beam. Current models range from a simple collection of lenses and masks to complex robots with components that control not only the laser parameters but also the optical and mechanical components. Because the process is dealing with submicron (less than 0.00001 of a meter) accuracy, great demands are placed upon such systems for stability, even though the interaction of the laser and tissue lasts only microseconds.
Using the system requires exquisite technical and biological control to modulate corneal shaping.
Another laser treatment process focuses light, like a magnifying glass, to boil away tissue one cell at a time, instead of carving away the surface. One problem is adequate control to prevent the process from cutting through a layer of corneal tissue known as Bowman's membrane--a section of the eye that does not regenerate.